1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to improved line interface circuitry providing improved noise immunity and lower cost at a central office of a telecommunications system.
2. Discussion of the Related Art
In recent years telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications includes not only the transmission of voice information, but also PSTN (public switched telephone network) modem information, control signals, and other information that is transmitted in the POTS bandwidth, which extends from approximately DC to approximately 3.4 kilohertz.
New, high-speed data communications provided over digital subscriber lines, such as Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), etc. (more broadly denoted as xDSL) provide for high speed data transmissions, as is commonly used in communicating over the Internet. As is known, the bandwidth for xDSL transmissions is generally defined by a lower cutoff frequency of approximately 30 kilohertz, and a higher cutoff frequency which varies depending upon the particular technology. Since the POTS and xDSL signals are defined by isolated frequency bands, both signals may be transmitted over the same two-wire loop.
A POTS splitter is typically provided at each end of a transmission system communicating both POTS and xDSL information. As is known, a POTS splitter circuit consists of a high-pass filter and a low-pass filter, which are used to separate the two signals (the POTS signal from the xDSL signal). Normally, the high-pass filter is built into the xDSL transceiver, whereas the low-pass filter (commonly referred to as a POTS filter) is normally provided as a separate unit. In operation, the POTS filter operates to reject the high frequency xDSL signals in order to protect the POTS (e.g., telephone) circuitry.
At the same time the POTS filter provides filtering and protection for higher frequency noise signals that are often associated with a ring signal, or the switching circuitry of a central office.
A "line-card", containing line interface circuitry, is provided at the central office.
The line interface circuitry provides the interconnections among the xDSL circuitry, POTS or PSTN voice circuitry, off-hook (or tip/ring) detection circuitry, ring generator circuitry, and the local loop. The line interface circuitry also includes the appropriate electronic filtering circuitry to minimize any noise transmission across the local loop. As is known, there are several such sources of noise, including the ring generator circuitry, and contact noise that results from switch contacts or other mechanisms for opening and closing circuit connections.
Conventionally, in prior art systems a POTS filter is interposed between the various POTS circuits and the xDSL circuit. This filter protects the POTS circuitry from the high frequency signals of the xDSL transmission circuitry, as well as filters noise generated from the relay or other circuit contacts and the ring generator circuitry from reaching the local loop. To achieve these objectives, this filter circuit is typically realized by a relatively high-order filter to achieve the desired characteristics. Namely, to achieve a cut-off frequency in the range of 4 to 10 kilohertz, but achieve a very large stop band attenuation (e.g., greater than 100 dB) inside the xDSL frequency band (i.e., above 30 kilohertz). As is known, the requirement of a high-order filter complicates the filter design and thus makes the filter more expensive.